The main stream of the photoreceptor to be used in the electrophotography has been changed from an inorganic photoreceptor to an organic photoreceptor which has advantages such as reducing of environment contamination and ease of the production. Therefore, organic photoreceptors using various materials have been developed.
Recently, function separated type photoreceptors are mainly used in which different materials are each separately put in charge of the function of charge generation and that of charge transportation. Among then, a multi-layered type photoreceptor is widely used, in which a charge generation layer and a charge transportation layer are laminated.
Besides, in the image forming process, the image forming method can be roughly classified into an analogical method using a halogen lamp as the light source and a digital method using a LED or laser light source. The digital latent image forming method is rapidly become to the main stream of both of the printer for forming a hardcopy by a personal computer and a copy machine for common use since such the method is easily applied for image processing and for combined image forming machine.
In the digital image formation, a laser, particularly a semiconductor laser or a LED, is used as the light source for writing image information converted to digital electric signals as a static latent image on the photoreceptor. As to the image formation by the laser light, a peculiar problem of formation of interference fringes has been known which is caused by the reflection of the light at the surface of the photoreceptor.
Moreover, the writing speed is lowered in the writing by the digital method since the diameter of the light beam for writing is small. Therefore, a reversal development is mainly applied for developing the exposed area. It has been known that a problem of formation of a black spot caused by a local defect of the photoreceptor is peculiarly accompanied with the reversal development. The formation of the black spot is a phenomenon that the toner is adhered to form fogging at a portion to be made as a white background of the image.
On the other hand, a belt type photoreceptor is proposed and practically used as the electrophotographic photoreceptor. The belt type photoreceptor is utilized for a high speed or color image forming apparatus since the belt type photoreceptor is flexible so as to have a high freeness of design and the durability of it can be made larger than that of the drum type photoreceptor. Furthermore, it is proposed to apply the belt type photoreceptor to a compact apparatus by making small the suspending roller of the photoreceptor belt.
However, the belt type photoreceptor suffers considerably serious force by the stress caused by the curvature of roller such as the driving roller and the suspension roller and that caused by the tension while driving and standing. Accordingly, peeling off of the jointed portion of the photoreceptor belt and scattering of the powder of the binder and cracking of the photoreceptive layer are tend to be occurred in the course of repeated used since the adhesive force between the support or the lower layer is weak. As a result of that, a problem of the image defect formation causing the black spot is easily to be occurred.
Besides, it has been recently required from the viewpoint of the space saving to make compact the electrophotographic image forming apparatus such as the copy machine and the printer to be used in an office. The electrophotographic image forming apparatus is generally constituted by a charging means, a developing means, a transfer means, a cleaning means and a discharging means each arranged around the photoreceptor. Therefore, the size of the electrophotographic image forming apparatus is strongly depended on the diameter of the photoreceptor. It is necessary to make small the diameter of the photoreceptor arranged at the center of the apparatus for making compact the electrophotographic image forming apparatus. Consequently, the proposition of the photoreceptor having a small diameter is demanded. The thickness of the layer of the organic photoreceptor is generally at least 17 μm. It has been tried to make larger the dry thickness of the photoreceptive layer for extending the durability or life of photoreceptor. However, the thickened layer causes a problem that the adhesion ability between the support and the photoreceptive layer or an interlayer and the photoreceptive layer is degraded since the interior stress in the photoreceptive layer is increased accompanied with the increasing the layer thickness. The adhesion ability is lowered accompanied with increasing of the layer thickness and decreasing of the diameter of the cylindrical support. Consequently, the peeling off of the photoreceptive layer is occurred in the course of the repeating use when the diameter of the photoreceptor is simply made small. Such the tendency is become considerable in the photoreceptor having a diameter of 50 mm or less.
For improving the adhesion ability, methods have been known such as roughing the support surface, arranging an adhesive layer between the photoreceptive layer and the support, and raising the adhesion ability of the charge generation layer when the photoreceptive layer comprises a piled layer of the charge generation layer and the charge transportation layer, have been known. These methods, however, cannot improve the durability of the photoreceptor since such the methods give an undesirable effect on the static or photographic property of the photoreceptor.
Japanese Patent Publication to Open for Public Inspection, hereinafter referred to as JP O.P.I., No. 03-179362 describes a method by which a cell structure, Benard cell, is formed in the subbing layer for roughing the surface thereof but the method causes an image defect since the effect of the method cannot be controlled. JP O.P.I. No. 08-248651 described that the leveling property of the subbing layer is degraded by the formation of the Benard cell at the time of immersion coating of and the electrophotographic property is lowered. Generally, the formation of the Benard cell has been considered as an undesirable matter and reduction of the Benard cell has been tried.
Although JP O.P.I. Nos. 60-32056 and 60-252359 positively describe a electroconductive layer and an interlayer each having the Benard cell, the object of the investigation is a countermeasure to moire, and there is no description regarding the relation to the black spot formation or the improvement on the electrophotographic property.